This invention relates to a hot water heating system of the general type disclosed in British Pat. Nos. 1,180,462 and 1,180,463. In previous systems a plurality of heat distributing units are arranged in a room or group of rooms to be heated and hot water is passed through them in series to heat each in sequence. Unless suitable Tees or bypasses are installed, no individual unit can be closed off without shutting down the entire system. In the aforesaid British patents, hot water is circulated from a manifold through each radiator or convector and then back to the manifold for return to the boiler. Each radiator or convector is equipped with a valve so that circulation through each individual unit may be restricted or shut off completely. However, where low water content boilers are used as the heat source, it is necessary to have flow through at least one unit so as to provide for circulation back to the pump to prevent damage to the pump as well as to the boiler.